Techniques for processing a target return signal using free-space optics

ABSTRACT

Free-space optics for use in a light detection and ranging (LIDAR) apparatus include a polarization beam-splitter (PBS) to direct an optical beam in a first direction toward a target environment and to propagate a portion of the optical beam in a second direction for receipt by a photodetector (PD), a polarization wave plate (PWP) to convert the optical beam from a first polarization to a second polarization, and to convert the target return signal from a third polarization to a fourth polarization, and a lens system coupled between the PBS and the PWP to magnify the optical beam. The propagated portion of the optical beam comprises a local oscillator (LO) signal to mix with a target return signal to generate target information.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.17/481,142 filed Sep. 21, 2021, which is a continuation of U.S. patentapplication Ser. No. 16/415,836 filed May 17, 2019, the entire contentsof each of which are hereby incorporated by reference.

TECHNICAL FIELD

The present disclosure relates generally to light detection and ranging(LIDAR) that provides simultaneous measurement of range and velocityacross two dimensions.

BACKGROUND

Fast-scanning mirrors and pulsed or frequency swept optical sources arethe primary components used to illuminate a target environment in mostconventional LIDAR systems. One mirror typically scans quickly along theX direction (azimuth), while another mirror scans more slowly along theY direction (elevation). Light emission and detection from targetreflections are done coaxially, typically via one or more single modewaveguides in a photonic integrated circuit or in single mode fibers.The light collected from the illuminated target has a measured delaythat is used to extract range information, and a frequency shift thatcan be used to extract velocity information. A 3-dimensional point cloudcan be established when the point-wise detected range information iscombined with angular position data from the scanning mirrors. Toachieve higher frame rates, the angular velocity of the scanning mirrorcan be increased, especially that of the scanner in the faster scandirection. When using a mirror with a high angular velocity andfiber-based detection, the target signal from distant objects isdegraded. One source of signal degradation is a loss of angular accuracydue to the change in the angular position of the scanner mirror duringthe round-trip time of the optical signal to and from a distant target.This error is usually referred to as “insufficient de-scan” or“walk-off” A secondary source of signal degradation is due to opticalaberrations which can distort/expand the return beam, reducing thecoupling efficiency on the small waveguide apertures or fiber tip.

SUMMARY

The present disclosure includes, without limitation, the followingexamples:

One example of a light detection and ranging (LIDAR) apparatus includesan optical source configured to emit an optical beam, a photodetectorconfigured to receive a target return signal, and free-space optics thatincludes a polarizing beam-splitter configured to transmit the opticalbeam in a first direction toward a target environment and to leak aportion of the optical beam in a second direction toward thephotodetector, where the leaked portion of the optical beam serves as alocal oscillator signal to mix with the target return signal to generatetarget information.

Another example of a LIDAR apparatus includes an optical sourceconfigured to emit an optical beam, a photodetector configured toreceive a target return signal, and free-space optics that include apolarizing beam-splitter to transmit the optical beam in a firstdirection toward a target environment, and a polarization wave plate totransmit the optical beam toward the target environment and to reflect aportion of the optical beam to the polarizing beam-splitter, where thereflected portion of the optical beam is directed in a second directiontoward the photodetector, serving as a local oscillator signal to mixwith the target return signal to generate target information.

One example method in a LIDAR apparatus includes generating an opticalbeam from an optical source having a first linear polarization;directing the optical beam in a first direction, toward a targetenvironment, with a polarizing beam splitter; leaking a portion of theoptical beam with the first linear polarization, from the polarizingbeam splitter in a second direction, toward a photodetector as a localoscillator signal; receiving, at the photodetector, the local oscillatorsignal and a target return signal having the first linear polarization;and mixing the target return signal with the LO signal to generatetarget information.

Another example method in a LIDAR apparatus includes generating anoptical beam from an optical source having a first linear polarization;directing the optical beam in a first direction, toward a targetenvironment, with a polarizing beam splitter; reflecting a portion ofthe optical beam as a local oscillator signal having a second linearpolarization, toward the PBS from a polarization wave plate; receiving,at a photodetector, the local oscillator signal and a target returnsignal having the second linear polarization; and mixing the targetreturn signal with the local oscillator signal to generate targetinformation.

These and other aspects of the present disclosure will be apparent froma reading of the following detailed description together with theaccompanying figures, which are briefly described below. The presentdisclosure includes any combination of two, three, four or more featuresor elements set forth in this disclosure, regardless of whether suchfeatures or elements are expressly combined or otherwise recited in aspecific example implementation described herein. This disclosure isintended to be read holistically such that any separable features orelements of the disclosure, in any of its aspects and examples, shouldbe viewed as combinable unless the context of the disclosure clearlydictates otherwise.

It will therefore be appreciated that this Summary is provided merelyfor purposes of summarizing some examples so as to provide a basicunderstanding of some aspects of the disclosure without limiting ornarrowing the scope or spirit of the disclosure in any way. Otherexample implementations, aspects, and advantages will become apparentfrom the following detailed description taken in conjunction with theaccompanying figures which illustrate the principles of the describedexamples.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of various examples, reference is nowmade to the following detailed description taken in connection with theaccompanying drawings in which like identifiers correspond to likeelements:

FIG. 1 is a block diagram illustrating an example LIDAR system accordingto the present disclosure;

FIG. 2 is a block diagram illustrating an example single-beam LIDARapparatus in a first configuration according to the present disclosure;

FIG. 3 is a block diagram illustrating an example multi-beam LIDARapparatus in a first configuration according to the present disclosure;

FIG. 4 is a block diagram illustrating an example single-beam LIDARapparatus in a second configuration according to the present disclosure;

FIG. 5 is a block diagram illustrating an example multi-beam LIDARapparatus in a second configuration according to the present disclosure;

FIG. 6 is a block diagram illustrating an example single-beam LIDARapparatus in a third configuration according to the present disclosure;

FIG. 7 is a block diagram illustrating an example multi-beam LIDARapparatus in a third configuration according to the present disclosure;

FIG. 8 is a flow diagram illustrating an example method in a LIDARapparatus according to the present disclosure; and

FIG. 9 is a flow diagram illustrating another example method in a LIDARapparatus according to the present disclosure.

DETAILED DESCRIPTION

The present disclosure describes examples of frequency-modulatedcontinuous-wave (FMCW) LIDAR apparatus, and example methods therein,that incorporate free-space optical components (free-space optics)coupled to an optical source, which may be a coherent or non-coherentoptical source and which may be a single-beam or multi-beam opticalsource, implemented in a photonic integrated circuit (PIC) or withdiscrete components, for example. The free-space optics may include,without limitation, polarizing beam splitters, lens systems,polarization wave plates, wavelength demultiplexers, reflectors andfree-space photodetectors.

The free-space optics deliver an optical beam to the target environment,and generate and mix the local oscillator signal with the target returnsignal on the photodetector. The free-space optics design eliminates thechallenging task of integrating the aforementioned components into aphotonics chip.

The free-space design also increases the collection efficiency of thetarget return signal and produces a higher signal-to-noise ratio (SNR)than conventional integrated designs through the use of large apertureoptics and photodetectors. In general, the efficiency of combining thetarget return signal with the LO signal is based on the spatial overlapbetween the LO signal and the target return signal on the photodetector.Examples in the present disclosure address the deficiencies of aconventional integrated LIDAR system by combining the LO signal and thetarget return signal in free space and providing the combined signal toa large aperture photodetector. In the free space optics, the targetsignal interferes with the LO signal to form the combined signal. Thefree-space design provides a large active surface area for mixing thesignals compared with conventional integrated on-chip LIDAR designs. Inaddition to relaxing the alignment requirements, the large activesurface area compensates for the time-dependent deleterious effects oflag angle and beam aberrations typically seen in fast scanning LiDARsystems.

FIG. 1 illustrates a LIDAR system 100 according to exampleimplementations of the present disclosure. The LIDAR system 100 includesone or more of each of a number of components, but may include fewer oradditional components than shown in FIG. 1 . The LIDAR system 100 may beimplemented in any sensing market, such as, but not limited to,transportation, manufacturing, metrology, medical, and security systems.For example, in the automotive industry, the described beam deliverysystem becomes the front-end of frequency modulated continuous-wave(FMCW) devices that can assist with spatial awareness for automateddriver assist systems, or self-driving vehicles. As shown, the LIDARsystem 100 includes optical circuits 101 which may be implemented on aphotonics chip. The optical circuits 101 may include a combination ofactive optical components and passive optical components. Active opticalcomponents may generate, amplify, or detect optical signals and thelike. In some examples, the active optical components may generate,amplify or detect optical beams at different wavelengths. Passiveoptical components may filter, attenuate, guide, reflect or alter thepolarization of optical beams and the like. In some examples, passiveoptical components such as wavelength demultiplexers may separateoptical beams of different wavelengths.

Free space optics 115 may include off-chip passive optical componentssuch as lens systems to collimate and expand optical beams, polarizingbeam splitters, polarization filters, polarization wave plates such asquarter-wave plates and half-wave plates to carry optical signals, androute and manipulate optical signals to appropriate input and outputports of the active optical circuits.

Optical scanner 102 may include one or more scanning mirrors that arerotatable along respective orthogonal axes to steer optical signals toscan a target environment according to a scanning pattern. For example,the scanning mirrors may be rotatable by one or more galvanometers. Theoptical scanner 102 may also collect light incident upon any objects inthe environment into a return optical beam (target return signal) thatis returned to the free-space optics 115. In addition to the mirrors andgalvanometers, the optical scanning system may include components suchas quarter-wave plates and half-wave plates, lenses, and anti-reflectivecoated windows or the like.

To control and support the optical circuits 101 and the optical scanner102, the LIDAR system 100 may include a LIDAR control system 110. TheLIDAR control system 110 may include a processing device for the LIDARsystem 100. In some examples, the processing device may be one or moregeneral-purpose processing devices such as a microprocessor, centralprocessing unit, or the like. More particularly, the processing devicemay be a complex instruction set computing (CISC) microprocessor,reduced instruction set computer (RISC) microprocessor, very longinstruction word (VLIW) microprocessor, or processor implementing otherinstruction sets, or processors implementing a combination ofinstruction sets. The processing device may also be one or morespecial-purpose processing devices such as an application specificintegrated circuit (ASIC), a field programmable gate array (FPGA), adigital signal processor (DSP), network processor, or the like.

In some examples, the LIDAR control system 110 may include a signalprocessing unit 112 such as a digital signal processor. The LIDARcontrol system 110 may be configured to output digital control signalsto control optical drivers 103. In some examples, the digital controlsignals may be converted to analog signals through signal conversionunit 106. For example, the signal conversion unit 106 may include adigital-to-analog converter. The optical drivers 103 may then providedrive signals to active components of optical circuits 101 to driveoptical sources such as lasers and amplifiers. In some embodiments,several optical drivers 103 and signal conversion units 106 may beprovided to drive multiple optical sources.

The LIDAR control system 110 may also be configured to output digitalcontrol signals for the optical scanner 102. A motion control system 105may control the galvanometers of the optical scanner 102 based oncontrol signals received from the LIDAR control system 110. For example,a digital-to-analog converter may convert coordinate routing informationfrom the LIDAR control system 110 to signals interpretable by thegalvanometers in the optical scanner 102. In some examples, a motioncontrol system 105 may also return information to the LIDAR controlsystem 110 about the position or operation of components of the opticalscanner 102. For example, an analog-to-digital converter may in turnconvert information about the galvanometers' position to a signalinterpretable by the LIDAR control system 110.

The LIDAR control system 110 may be further configured to analyzeincoming digital signals. In this regard, the LIDAR system 100 mayinclude optical receivers 104, such as free-space photodetectors, tomeasure one or more beams received by free-space optics 115. Forexample, a free-space photodetector may measure the amplitude of areference beam (e.g., a local oscillator signal) from the free-spaceoptics 115, and an analog-to-digital converter may convert signals fromthe optical receivers 104 to signals interpretable by the LIDAR controlsystem 110. The optical receivers 104 may also measure the opticalsignal (e.g., a target return signal) that carries information about therange and velocity of a target in the form of a beat frequency betweenthe local oscillator signal and the target return signal. The opticalreceivers 104 may include a high-speed analog-to-digital converter toconvert signals received by the optical receivers to signalsinterpretable by the LIDAR control system 110.

In some examples, the LIDAR system 100 may additionally include one ormore imaging devices 108 configured to capture images of the targetenvironment, a global positioning system 109 configured to provide ageographic location of the system 100, or other sensor inputs. The LIDARsystem 100 may also include an image processing system 114. The imageprocessing system 114 may be configured to receive the images andgeographic location, and send the images and location or informationrelated thereto to the LIDAR control system 110 or other systemsconnected to the LIDAR system 100.

In some examples, the LIDAR system 100 may be configured to usenondegenerate optical sources to simultaneously measure range andvelocity across two dimensions, providing for real-time, long rangemeasurements of range, velocity, azimuth, and elevation of thesurrounding environment. In some examples, the system may directmultiple modulated optical beams to the same target environment.

In some examples, the scanning process begins with the optical drivers103 and LIDAR control system 110. The LIDAR control system 110 instructsthe optical drivers 103 to independently modulate one or more opticalbeams, and these modulated signals propagate through the on-chip opticalcircuits, to and through the free-space optics 115. The free-spaceoptics 115 direct the optical beams at the optical scanning system 102that scans the target environment over a preprogrammed pattern definedby the motion control subsystem 105.

Optical signals reflected back from the target environment (the targetreturn signals) pass through the passive free-space optics 115 to thefree-space optical receivers 104. Each return signal is time-shifted inproportion to the target range, producing a constant, range-related beatfrequency when mixed with the frequency modulated reference signal(local oscillator signal), which is detected on the free-space opticalreceivers 104. Any relative velocity component of an illuminated target(relative to the LIDAR system 100) produces an additional Dopplerfrequency shift, proportional to the relative target velocity, that canbe distinguished from the range-related frequency offset by a signalprocessing unit 112 described below. The configurations of thefree-space optics 115 for polarizing and directing beams to the opticalreceivers 104 are described in detail below.

The analog signals from the optical receivers 104 are converted todigital signals using ADCs. The digital signals are then sent to theLIDAR control system 110. A signal processing unit 112 may then receivethe digital signals and interpret them. In some examples, the signalprocessing unit 112 also receives position data from the motion controlsystem 105 and galvanometer (not shown) as well as image data from theimage processing system 114. The signal processing unit 112 can thengenerate a 3D point cloud with information about range and velocity ofpoints in the environment as the optical scanner 102 scans additionalpoints. The signal processing unit 112 can also overlay a 3D point clouddata with the image data to determine velocity and distance of objectsin the surrounding area. The system also processes the satellite-basednavigation location data to provide a precise global location.

FIG. 2 is a block diagram illustrating an example single-beam LIDARapparatus 200 in a first configuration. The configuration illustrated inFIG. 2 includes a photonics chip 201, including an FMCW LIDAR photonicintegrated circuit (PIC) 202, and free-space optics. In one example, theoptical circuits 101, described in respect to FIG. 1 , may beimplemented on photonic chip 201. The PIC 202 is configured to generateand emit an optical beam 203, which may be multi-spectral (i.e.,containing more than one wavelength). The PIC 202 is optically coupledto a polarizing beam-splitter (PBS) 204. The PBS 204 transmits ap-polarization of the optical beam 203 in a first direction toward atarget 205 in the target environment. The p-polarization of the opticalbeam 203 is the polarization of the beam that is parallel to the planeof incidence of the optical beam 203 in the PBS 204. The PBS 204 isdesigned to have a finite polarization extinction ratio, such that adetectable p-polarized portion 206 of the optical beam 203 is leaked bythe PBS 204 in a second direction toward a photodetector 207. In oneexample, without limitation, the PBS 204 may have a polarizationextinction ratio on the order of 1:1000. The leaked portion 206 of theoptical beam 203 may be used as a local oscillator (LO) signal to mixwith a target return signal as described in greater detail below.

In one example, the free-space optics may include a lens system 208 tomagnify the optical beam 203. The lens system 208 may be any suitablelens system such as a Galilean or a Keplerian lens system, for example.

In one example, the free-space optics may include a polarization waveplate (PWP) 209, which may be a quarter-wave plate or half-wave plate,to convert the optical beam 203 from the first linear polarization(p-polarization) to a first circular polarization. In the exampleconfiguration of FIG. 2 , the first circular polarization is shown as aright-hand (RH) or clockwise (CW) circular polarization. The circularlypolarized optical beam 203 is then directed to the target environment byoptical scanners 210. In the example illustrated in FIG. 2 , the opticalbeam 203 illuminates the target 205, which reflects the optical beam asa target return signal 211. The principle component of the target returnsignal 211 will be a circularly polarized signal (second circularpolarization) with the opposite polarization sense of the circularlypolarized optical beam 203. In the example of FIG. 2 , the target returnsignal will have a left-hand (LH) or counter-clockwise (CCW) circularpolarization.

The target return signal 211 is de-scanned by the optical scanner 210and transmitted through the PWP 209, where the polarization of thetarget return signal 211 is converted from the second circularpolarization to an s-polarized signal (second linear polarization),perpendicular to the p-polarization (first linear polarization) of theoriginal optical beam 203. The s-polarized target return signal 211 thenpasses through the lens system 208 and is reflected by the PBS 204 in athird direction to a second PWP 212, which may be a quarter-wave plateor half-wave plate, to convert the target return signal 211 from thesecond linear polarization (s-polarization) to the first circularpolarization (RH or CW circular polarization in the example of FIG. 2 ).The circularly polarized target return signal 211 is then reflected byretro-reflector 213 back through the second PWP 212. The retro-reflector211 reverses the polarization sense of the target return signal 211 fromthe first circular polarization (RH or CW in the example of FIG. 2 ) tothe second circular polarization (LH or CCW in the example of FIG. 2 ),and the second PWP 212 converts the target return signal 211 from thesecond circular polarization to the first linear polarization(p-polarization in the example of FIG. 2 ).

The p-polarized target return signal 211 passes through PBS 204 toco-propagate with the LO signal 206 (leakage signal), where thep-polarized target return signal 211 and the p-polarized LO signal 206pass through a linear polarizer 214. The linear polarizer passes thep-polarized light and rejects any signal that is not p-polarized, whichprevents light of different polarizations from elevating the noise floorof the detection system.

For multiple wavelength beams, wavelength demultiplexers may be used toseparate different wavelengths and direct them to dedicated detectors.FIG. 2 illustrates an example of a two-wavelength configuration, where awavelength demultiplexer (DEMUX) 215 separates the first wavelength, λ₁,from the second wavelength, λ₂. The DEMUX 215 may be for example, andwithout limitation, a dichroic mirror, a Bragg grating or any othersuitable wavelength demultiplexer. The separate wavelengths may then befocused by respective lens systems 216 and 217 onto respectivephotodetectors 207 and 218, where the interference between the LO signal206 and the target return signal 211 generates target information asdescribed above. The use of large aperture free-space optics andfree-space photodetectors ensures that any beam decentering caused byinsufficient de-scan does not degrade the SNR of the target returnsignal 211.

FIG. 3 is a block diagram illustrating an example multi-beam LIDARapparatus in a first configuration. LIDAR apparatus 300 is similar inalmost all respects to LIDAR apparatus 200, except that the photonicschip 301 and the FMCW LIDAR PIC 302 are configured to emit multiplebeams, where each beam may be multi-spectral. In one example, theoptical circuits 101, described in respect to FIG. 1 , may beimplemented on photonics chip 301. As illustrated in FIG. 3 , theexample LIDAR apparatus 300 also includes a lens array 318 to collimatethe multiple beams into a collimated optical beam 303, which is directedto the PBS 204. The PBS 204 transmits a p-polarization of the opticalbeam 303 in a first direction toward the target 205 in the targetenvironment. As described above, due to the finite extinction ratio ofthe PBS 204, a detectable p-polarized portion 306 of the optical beam303 is leaked by the PBS 204 in a second direction toward photodetector207. The leaked portion 306 of the optical beam 303 may be used as alocal oscillator (LO) signal to mix with a target return signal. Thep-polarized optical beam 303 is then magnified by the lens system 208.

The optical beam 303 is then converted by the PWP 209 from the firstlinear polarization (p-polarization) to a first circular polarization.In the example configuration of FIG. 3 , the first circular polarizationis shown as a right-hand (RH) or clockwise (CW) circular polarization.The circularly polarized optical beam 303 is then directed to the targetenvironment by optical scanners 210. In the example illustrated in FIG.3 , the optical beam 303 illuminates the target 205, which reflects theoptical beam as a target return signal 311. The principle component ofthe target return signal 311 will be a circularly polarized signal(second circular polarization) with the opposite polarization sense ofthe circularly polarized optical beam 303. In the example of FIG. 3 ,the target return signal will have a left-hand (LH) or counter-clockwise(CCW) circular polarization.

The target return signal 311 is de-scanned by the optical scanner 210and transmitted through the PWP 209, where the polarization of thetarget return signal 311 is converted from the second circularpolarization to an s-polarized signal (second linear polarization),perpendicular to the p-polarization (first linear polarization) of theoriginal optical beam 303. The s-polarized target return signal 311 thenpasses through the lens system 208 and is reflected by the PBS 204 in athird direction to a second PWP 212, to convert the target return signal311 from the second linear polarization (s-polarization) to the firstcircular polarization (RH or CW circular polarization in the example ofFIG. 3 ). The circularly polarized target return signal 311 is thenreflected by retro-reflector 213, back through the second PWP 212. Theretro-reflector 211 reverses the polarization sense of the target returnsignal 311 from the first circular polarization (RH or CW in the exampleof FIG. 3 ) to the second circular polarization (LH or CCW in theexample of FIG. 3 ), and the second PWP 212 converts the target returnsignal 311 from the second circular polarization to the first linearpolarization (p-polarization in the example of FIG. 3 ).

The p-polarized target return signal 311 passes through PBS 204 toco-propagate with the LO signal 306 (leakage signal), where thep-polarized target return signal 311 and the p-polarized LO signal 306pass through linear polarizer 214. The linear polarizer passes thep-polarized light and rejects any signal that is not p-polarized, whichprevents light of different polarizations from elevating the noise floorof the detection system.

For multiple wavelength beams, wavelength demultiplexers may be used toseparate different wavelengths and direct them to dedicated detectors.FIG. 3 illustrates an example of a two wavelength configuration, where awavelength demultiplexer 215 separates the first wavelength, λ₁, fromthe second wavelength, λ₂. In the multi-beam system illustrated by FIG.3 , although not shown, multiple wavelength demultiplexers may be usedto spatially separate the multiple beams. The separate wavelengths maythen be focused by respective lens systems 216 and 217 onto respectivephotodetectors 207 and 218, where the interference between the LO signal306 and the target return signal 311 generates target information asdescribed above. The use of large aperture free-space optics andfree-space photodetectors insures that any beam decentering caused byinsufficient de-scan does not degrade the SNR of the target returnsignal 311.

FIG. 4 is a block diagram illustrating an example single-beam LIDARapparatus in a second configuration. The configuration illustrated inFIG. 4 is similar in many respects to the single-beam LIDAR apparatusillustrated in FIG. 2 , except that the scan port of the PBS 204 and thedetection port of the PBS 204 are reversed.

In FIG. 4 , the LIDAR apparatus 400 includes a photonics chip 201including an FMCW LIDAR photonic integrated circuit (PIC) 202 configuredto generate and emit an optical beam 403, which may be multi-spectral.In one example, the optical circuits 101, described in respect to FIG. 1, may be implemented on photonics chip 201. The PIC 202 is opticallycoupled to the polarizing beam-splitter (PBS) 204. The PBS 204 reflectsan s-polarization of the optical beam 403 in a first direction toward atarget 205 in the target environment. The s-polarization of the opticalbeam 403 is the polarization of the beam that is perpendicular to theplane of incidence of the optical beam 403 in the PBS 204. The PBS 204has a finite polarization extinction ratio, such that a detectables-polarized portion 406 of the optical beam 403 is leaked by the PBS 204in a second direction toward photodetector 207. The leaked portion 406of the optical beam 403 may be used as a local oscillator (LO) signal tomix with a target return signal. In one example, the free-space opticsmay include a lens system 208 to magnify the optical beam 403.

The free-space optics may include a polarization wave plate (PWP) 209,to convert the optical beam 403 from the first linear polarization(s-polarization) to a first circular polarization. In the exampleconfiguration of FIG. 4 , the first circular polarization is shown as aright-hand (RH) or clockwise (CW) circular polarization. The circularlypolarized optical beam 403 is then directed to the target environment byoptical scanners 210. In the example illustrated in FIG. 4 , the opticalbeam 403 illuminates the target 205, which reflects the optical beam asa target return signal 411. The principle component of the target returnsignal 411 will be a circularly polarized signal (second circularpolarization) with the opposite polarization sense of the circularlypolarized optical beam 403. In the example of FIG. 4 , the target returnsignal will have a left-hand (LH) or counter-clockwise (CCW) circularpolarization.

The target return signal 411 is de-scanned by the optical scanner 210and transmitted through the PWP 209, where the polarization of thetarget return signal 411 is converted from the second circularpolarization to a p-polarized signal (second linear polarization),perpendicular to the s-polarization (first linear polarization) of theoriginal optical beam 403. The p-polarized target return signal 411 thenpasses through the lens system 208 and is passed by the PBS 204 in athird direction to the second PWP 212, to convert the target returnsignal 411 from the second linear polarization (p-polarization) to thesecond circular polarization (LH or CCW circular polarization in theexample of FIG. 4 ). The circularly polarized target return signal 411is then reflected by retro-reflector 213 back through the second PWP212. The retro-reflector 211 reverses the polarization sense of thetarget return signal 411 from the second circular polarization (LH orCCW in the example of FIG. 4 ) to the first circular polarization (RH orCW in the example of FIG. 4 ), and the second PWP 212 converts thetarget return signal 411 from the first circular polarization to thefirst linear polarization (s-polarization in the example of FIG. 4 ).

The s-polarized target return signal 411 passes through PBS 204 toco-propagate with the LO signal 406 (leakage signal), where thes-polarized target return signal 411 and the s-polarized LO signal 406pass through a linear polarizer 414. The linear polarizer 414 passes thes-polarized light and rejects any signal that is not s-polarized, whichprevents light of different polarizations from elevating the noise floorof the detection system.

For multiple wavelength beams, wavelength demultiplexers may be used toseparate different wavelengths and direct them to dedicated detectors.FIG. 4 illustrates an example of a two-wavelength configuration, where awhere a wavelength demultiplexer (DEMUX) 215 separates the firstwavelength, λ₁, from the second wavelength, λ₂. The DEMUX 215 may be forexample, and without limitation, a dichroic mirror, a Bragg grating orany other suitable wavelength demultiplexer. The separate wavelengthsmay then be focused by respective lens systems 216 and 217 ontorespective photodetectors 207 and 218, where the interference betweenthe LO signal 406 and the target return signal 411 generates targetinformation as described above. The use of large aperture free-spaceoptics and free-space photodetectors insures that any beam decenteringcaused by insufficient de-scan does not degrade the SNR of the targetreturn signal 411.

FIG. 5 is a block diagram illustrating an example multi-beam LIDARapparatus in a second configuration. LIDAR apparatus 500 is similar inalmost all respects to LIDAR apparatus 400, except that the photonicschip 501 and the FMCW LIDAR PIC 502 are configured to emit multiplebeams, where each beam may be multi-spectral. In one example, theoptical circuits 101, described in respect to FIG. 1 , may beimplemented on photonics chip 501. As illustrated in FIG. 5 , theexample LIDAR apparatus 500 includes a lens array 518 to collimate themultiple beams into a collimated optical beam 503, which is directed tothe PBS 204. The PBS 204 reflects an s-polarization of the optical beam503 in a first direction toward a target 205 in the target environment.The s-polarization of the optical beam 503 is the polarization of thebeam that is perpendicular to the plane of incidence of the optical beam503 in the PBS 204. The PBS 204 has a finite polarization extinctionratio, such that an s-polarized portion 506 of the optical beam 503 isleaked by the PBS 204 in a second direction toward photodetector 207.The leaked portion 506 of the optical beam 503 may be used as a localoscillator (LO) signal to mix with a target return signal. Thefree-space optics may include a lens system 208 to magnify the opticalbeam 503.

The free-space optics may include a polarization wave plate (PWP) 209,to convert the optical beam 503 from the first linear polarization(s-polarization) to a first circular polarization. In the exampleconfiguration of FIG. 5 , the first circular polarization is shown as aright-hand (RH) or clockwise (CW) circular polarization. The circularlypolarized optical beam 503 is then directed to the target environment byoptical scanners 210. In the example illustrated in FIG. 5 , the opticalbeam 503 illuminates the target 205, which reflects the optical beam asa target return signal 511. The principle component of the target returnsignal 511 will be a circularly polarized signal (second circularpolarization) with the opposite polarization sense of the circularlypolarized optical beam 503. In the example of FIG. 5 , the target returnsignal will have a left-hand (LH) or counter-clockwise (CCW) circularpolarization.

The target return signal 511 is de-scanned by the optical scanner 210and transmitted through the PWP 209, where the polarization of thetarget return signal 511 is converted from the second circularpolarization to a p-polarized signal (second linear polarization),perpendicular to the s-polarization (first linear polarization) of theoriginal optical beam 503. The p-polarized target return signal 511 thenpasses through the lens system 208 and is passed by the PBS 204 in athird direction to the second PWP 212, to convert the target returnsignal 511 from the second linear polarization (p-polarization) to thesecond circular polarization (LH or CCW circular polarization in theexample of FIG. 4 ). The circularly polarized target return signal 511is then reflected by retro-reflector 213 back through the second PWP212. The retro-reflector 211 reverses the polarization sense of thetarget return signal 511 from the second circular polarization (LH orCCW in the example of FIG. 5 ) to the first circular polarization (RH orCW in the example of FIG. 5 ), and the second PWP 212 converts thetarget return signal 511 from the first circular polarization to thefirst linear polarization (s-polarization in the example of FIG. 5 ).

The s-polarized target return signal 511 passes through PBS 204 toco-propagate with the LO signal 506 (leakage signal), where thes-polarized target return signal 511 and the s-polarized LO signal 506pass through a linear polarizer 514. The linear polarizer 514 passes thes-polarized light and rejects any signal that is not s-polarized, whichprevents light of different polarizations from elevating the noise floorof the detection system.

For multiple wavelength beams, wavelength demultiplexers may be used toseparate different wavelengths and direct them to dedicated detectors.FIG. 5 illustrates an example of a two-wavelength configuration, where awhere a wavelength demultiplexer (DEMUX) 215 separates the firstwavelength, λ₁, from the second wavelength, λ₂. The DEMUX 215 may be forexample, and without limitation, a dichroic mirror, a Bragg grating orany other suitable wavelength demultiplexer. In the multi-beam systemillustrated by FIG. 5 , although not shown, multiple wavelengthdemultiplexers may be used to spatially separate the multiple beams. Theseparate wavelengths may then be focused by respective lens systems 216and 217 onto respective photodetectors 207 and 218, where theinterference between the LO signal 506 and the target return signal 511generates target information as described above. The use of largeaperture free-space optics and free-space photodetectors insures thatany beam decentering caused by insufficient de-scan does not degrade theSNR of the target return signal 511.

FIG. 6 is a block diagram illustrating an example single-beam LIDARapparatus in a third configuration. The LIDAR apparatus 600 illustratedin FIG. 6 includes a photonics chip 201, including an FMCW LIDARphotonic integrated circuit (PIC) 202, and free-space optics. In oneexample, the optical circuits 101, described in respect to FIG. 1 , maybe implemented on photonics chip 201. The PIC 202 is configured togenerate and emit an optical beam 603, which may be multi-spectral(i.e., containing more than one wavelength). The PIC 202 is opticallycoupled to a polarizing beam-splitter (PBS) 204. The PBS 204 transmits ap-polarization of the optical beam 603 in a first direction toward atarget 205 in the target environment. In one example, the free-spaceoptics may include a lens system 208 to magnify the optical beam 603.

In one example, the free-space optics includes a polarization wave plate(PWP) 209, which may be a quarter-wave plate or half-wave plate, toconvert the optical beam 603 from the first linear polarization(p-polarization) to a first circular polarization. In the exampleconfiguration of FIG. 6 , the first circular polarization is shown as aright-hand (RH) or clockwise (CW) circular polarization. The circularlypolarized optical beam 603 is then directed to the target environment byoptical scanners 210.

The PWP 209 is designed to partially reflect and change the polarizationof the p-polarized optical beam 603, such that a detectable s-polarizedportion 606 of the optical beam 603 is reflected back through the lenssystem 208 to the PBS 204, where it is reflected in a second directiontoward PD 207, and where it may be used as a local oscillator (LO)signal to mix with a target return signal as described below.

Continuing with the example illustrated in FIG. 6 , the circularlypolarized optical beam 603 illuminates the target 205, which reflectsthe optical beam as a target return signal 611. The principle componentof the target return signal 611 will be a circularly polarized signal(second circular polarization) with the opposite polarization sense ofthe circularly polarized optical beam 603. In the example of FIG. 6 ,the target return signal will have a left-hand (LH) or counter-clockwise(CCW) circular polarization.

The target return signal 611 is de-scanned by the optical scanner 210and transmitted through the PWP 209, where the polarization of thetarget return signal 611 is converted from the second circularpolarization to an s-polarized signal (second linear polarization),perpendicular to the p-polarization (first linear polarization) of theoriginal optical beam 603. The s-polarized target return signal 611 thenpasses through the lens system 208 and is reflected by the PBS 204 inthe second direction to co-propagate with the LO signal 606 (reflectedsignal), where the s-polarized target return signal 611 and thes-polarized LO signal 606 pass through a linear polarizer 614. Thelinear polarizer passes the s-polarized light and rejects any signalthat is not s-polarized, which prevents light of different polarizationsfrom elevating the noise floor of the detection system.

For multiple wavelength beams, wavelength demultiplexers may be used toseparate different wavelengths and direct them to dedicated detectors.FIG. 6 illustrates an example of a two-wavelength configuration, where awhere a wavelength demultiplexer (DEMUX) 215 separates the firstwavelength, λ₁, from the second wavelength, λ₂. The DEMUX 215 may be forexample, and without limitation, a dichroic mirror, a Bragg grating orany other suitable wavelength demultiplexer. The separate wavelengthsmay then be focused by respective lens systems 216 and 217 ontorespective photodetectors 207 and 218, where the interference betweenthe LO signal 606 and the target return signal 611 generates targetinformation as described above. The use of large aperture free-spaceoptics and free-space photodetectors insures that any beam decenteringcaused by insufficient de-scan does not degrade the SNR of the targetreturn signal 611.

FIG. 7 is a block diagram illustrating an example multi-beam LIDARapparatus in a third configuration. LIDAR apparatus 700 is similar inalmost all respects to LIDAR apparatus 600, except that the photonicschip 701 and the FMCW LIDAR PIC 702 are configured to emit multiplebeams, where each beam may be multi-spectral. In one example, theoptical circuits 101, described in respect to FIG. 1 , may beimplemented on photonics chip 701. As illustrated in FIG. 7 , theexample LIDAR apparatus 700 includes a lens array 718 to collimate themultiple beams into a collimated optical beam 703, which is directed tothe PBS 204. The PBS 204 transmits a p-polarization of the optical beam703 in a first direction toward a target 205 in the target environment.As described above, the free-space optics may include a lens system 208to magnify the optical beam 703.

In one example, the free-space optics include a polarization wave plate(PWP) 209, which may be a quarter-wave plate or half-wave plate, toconvert the optical beam 703 from the first linear polarization(p-polarization) to a first circular polarization. In the exampleconfiguration of FIG. 7 , the first circular polarization is shown as aright-hand (RH) or clockwise (CW) circular polarization. The circularlypolarized optical beam 703 is then directed to the target environment byoptical scanners 210.

The PWP 209 is designed to partially reflect and change the polarizationof the p-polarized optical beam 703, such that a detectable s-polarizedportion 706 of the optical beam 703 is reflected back through the lenssystem 208 to the PBS 204, where it is reflected in a second directiontoward PD 207, and where it may be used as a local oscillator (LO)signal to mix with a target return signal as described below.

Continuing with the example illustrated in FIG. 7 , the circularlypolarized optical beam 703 illuminates the target 205, which reflectsthe optical beam as a target return signal 711. The principle componentof the target return signal 711 will be a circularly polarized signal(second circular polarization) with the opposite polarization sense ofthe circularly polarized optical beam 603. In the example of FIG. 7 ,the target return signal will have a left-hand (LH) or counter-clockwise(CCW) circular polarization.

The target return signal 711 is de-scanned by the optical scanner 210and transmitted through the PWP 209, where the polarization of thetarget return signal 711 is converted from the second circularpolarization to an s-polarized signal (second linear polarization),perpendicular to the p-polarization (first linear polarization) of theoriginal optical beam 703. The s-polarized target return signal 711 thenpasses through the lens system 208 and is reflected by the PBS 204 inthe second direction to co-propagate with the LO signal 706 (reflectedsignal), where the s-polarized target return signal 711 and thes-polarized LO signal 706 pass through a linear polarizer 714. Thelinear polarizer passes the s-polarized light and rejects any signalthat is not s-polarized, which prevents light of different polarizationsfrom elevating the noise floor of the detection system.

For multiple wavelength beams, wavelength demultiplexers may be used toseparate different wavelengths and direct them to dedicated detectors.FIG. 6 illustrates an example of a two-wavelength configuration, where awhere a wavelength demultiplexer (DEMUX) 215 separates the firstwavelength, λ₁, from the second wavelength, λ₂. The DEMUX 215 may be forexample, and without limitation, a dichroic mirror, a Bragg grating orany other suitable wavelength demultiplexer. In the multi-beam systemillustrated by FIG. 7 , although not shown, multiple wavelengthdemultiplexers may be used to spatially separate the multiple beams. Theseparate wavelengths may then be focused by respective lens systems 216and 217 onto respective photodetectors 207 and 218, where theinterference between the LO signal 706 and the target return signal 711generates target information as described above. The use of largeaperture free-space optics and free-space photodetectors insures thatany beam decentering caused by insufficient de-scan does not degrade theSNR of the target return signal 711.

FIG. 8 is a flow diagram illustrating an example method 800 in a LIDARapparatus, according to the present disclosure. Various portions ofmethod 800 may be performed by LIDAR apparatus 200, 300, 400 and 500,illustrated in FIGS. 2, 3, 4 and 5 , respectively and described indetail above.

With reference to FIG. 8 , method 800 illustrates example functions usedby various embodiments. Although specific function blocks (“blocks”) aredisclosed in method 800, such blocks are examples. That is, embodimentsare well suited to performing various other blocks or variations of theblocks recited in method 800. It is appreciated that the blocks inmethod 800 may be performed in an order different than presented, andthat not all of the blocks in method 800 may be performed.

At block 802, an optical source of a LIDAR apparatus generates anoptical beam having a first linear polarization. In various examples,the optical light source may be any of photonic ICs 202, 302 and 502 asdescribed with respect to FIGS. 2-5 . The optical sources may generatemultiple beams, and each beam may have multiple wavelengths. In variousexamples the linear polarization may be p-polarization or s-polarizationwith respect to a PBS that receives the optical beam.

At block 804, the optical beam is directed by a PBS in the direction ofa target environment. In some examples, the optical beam may bep-polarized, and the PBS passes the optical beam toward the targetenvironment, as illustrated and described with respect to LIDARapparatus 200 and 300 in FIGS. 2 and 3 , respectively. In otherexamples, the optical beam may be s-polarized, and the PBS may reflectthe optical beam toward the target environment, as illustrated anddescribes with respect to LIDAR apparatus 400 and 500 in FIGS. 4 and 5 ,respectively.

At block 806, a portion of the optical beam is leaked from the PBS inthe direction of a photodetector (PD) as an LO signal. In some examples,the optical beam may be p-polarized, and the PBS reflects the leakedportion of the optical beam toward the PD, as illustrated and describedwith respect to LIDAR apparatus 200 and 300 in FIGS. 2 and 3 ,respectively. In other examples, the optical beam may be s-polarized,and the PBS passes the leaked portion of the optical beam toward the PD,as illustrated and described with respect to LIDAR apparatus 400 and 500in FIGS. 4 and 5 , respectively.

At block 808, the LO signal and the target return signal are received atthe PD with the same polarization. In some examples, the two signals mayboth be p-polarized as illustrated and described with respect to LIDARapparatus 200 and 300 in FIGS. 2 and 3 , respectively. In otherexamples, both signals may be s-polarized as illustrated and describedwith respect to LIDAR apparatus 400 and 500 in FIGS. 4 and 5 ,respectively.

At block 810, the LO signal and the target return signal are mixed togenerate target information. The mixing may occur at the PD or anywherealong the optical path where the two signals are co-propagating with thesame polarization.

FIG. 9 is a flow diagram illustrating an example method 900 in a LIDARapparatus, according to the present disclosure. Various portions ofmethod 900 may be performed by LIDAR apparatus 600 and 700, illustratedin FIGS. 6 and 7 , respectively, and described in detail above.

With reference to FIG. 9 , method 900 illustrates example functions usedby various embodiments. Although specific function blocks (“blocks”) aredisclosed in method 800, such blocks are examples. That is, embodimentsare well suited to performing various other blocks or variations of theblocks recited in method 900. It is appreciated that the blocks inmethod 900 may be performed in an order different than presented, andthat not all of the blocks in method 800 may be performed.

At block 902, an optical source of a LIDAR apparatus generates anoptical beam having a first linear polarization. In various examples,the optical light source may be either of photonic ICs 202 and 702 asdescribed with respect to FIGS. 6 and 7 , respectively. The opticalsources may generate multiple beams, and each beam may have multiplewavelengths. In various examples the linear polarization may bep-polarization or s-polarization with respect to a PBS that receives theoptical beam.

At block 904, the optical beam is directed by a PBS in the direction ofa target environment. In the examples of FIGS. 6 and 7 , the opticalbeam is p-polarized, and the PBS passes the optical beam toward thetarget environment. It will be appreciated, however, that the targetport and the detection port of apparatus 600 and 700 may beinterchanged, to provide for the use of s-polarization in the opticalbeam generated by PICs 202 or 702, in which case an s-polarized opticalbeam would be reflected toward the target environment by the PBS.

At block 906, a portion of the optical beam is reflected from a PWB togenerate an LO signal with a second linear polarization. In the examplesof LIDAR apparatus 600 and 700 in FIGS. 6 and 7 , the secondpolarization is illustrated as s-polarization. As illustrated in FIGS. 6and 7 , the LO signal co-propagates with a target return signal of thesame polarization all the way back to the PD.

At block 908, the LO signal and the target return signal are received atthe PD with the same polarization after co-propagating through apparatus600 and 700 from the PWP to the PD, as illustrated in FIGS. 6 and 7 ,respectively.

At block 910, the LO signal and the target return signal are mixed togenerate target information. The mixing may occur at the PD or anywherealong the optical path where the two signals are co-propagating with thesame polarization.

The preceding description sets forth numerous specific details such asexamples of specific systems, components, methods, and so forth, inorder to provide a good understanding of the present disclosure. It willbe apparent to one skilled in the art, however, that other examples maybe practiced without these specific details. In other instances,well-known components or methods are not described in detail or arepresented in simple block diagram form in order to avoid unnecessarilyobscuring the present disclosure. Thus, the specific details set forthare merely non-limiting examples.

Although the operations of the methods herein are shown and described ina particular order, the order of the operations of each method may bealtered so that certain operations may be performed in an inverse orderor so that certain operation may be performed, at least in part,concurrently with other operations. In other examples, instructions orsub-operations of distinct operations may be performed in anintermittent or alternating manner.

The above description of illustrated examples of the invention,including what is described in the Abstract, is not intended to beexhaustive or to limit the invention to the precise forms disclosed.While specific examples are described herein for illustrative purposes,various equivalent modifications are possible within the scope of thedisclosure, as those skilled in the relevant art will recognize. Theword “example” or is used herein to mean serving as an example,instance, or illustration. Any aspect or design described herein as“example” is not necessarily to be construed as preferred oradvantageous over other aspects or designs. Rather, use of the word“example” is intended to present concepts in a concrete fashion. As usedin this disclosure, the term “or” is intended to mean an inclusive “or”rather than an exclusive “or”. That is, unless specified otherwise, orclear from context, “X includes A or B” is intended to mean any of thenatural inclusive permutations. That is, if X includes A; X includes B;or X includes both A and B, then “X includes A or B” is satisfied underany of the foregoing instances. In addition, the articles “a” and “an”as used in this application and the appended claims should generally beconstrued to mean “one or more” unless specified otherwise or clear fromcontext to be directed to a singular form. Furthermore, the terms“first,” “second,” “third,” “fourth,” etc. as used herein are meant aslabels to distinguish among different elements and may not necessarilyhave an ordinal meaning according to their numerical designation.

What is claimed is:
 1. Free-space optics for use in a light detectionand ranging (LIDAR) apparatus, comprising: a polarization beam-splitter(PBS) to direct an optical beam in a first direction toward a targetenvironment and to propagate a portion of the optical beam in a seconddirection for receipt by a photodetector (PD), wherein the propagatedportion of the optical beam comprises a local oscillator (LO) signal tomix with a target return signal to generate target information; apolarization wave plate (PWP) to convert the optical beam from a firstpolarization to a second polarization, and to convert the target returnsignal from a third polarization to a fourth polarization; and a lenssystem coupled between the PBS and the PWP to magnify the optical beam.2. The free-space optics of claim 1, wherein the first polarization is afirst linear polarization and the second polarization is a secondcircular polarization, and wherein the third polarization is a thirdcircular polarization and the fourth polarization is a fourth linearpolarization.
 3. The free-space optics of claim 1, wherein the PWP is afirst PWP, and wherein the free-space optics further comprise: a secondPWP to convert the target return signal from the fourth polarization tothe second polarization; and a reflector to convert the target returnsignal from the second polarization to the third polarization, and toreflect the target return signal through the second PWP.
 4. Thefree-space optics of claim 1, wherein the PD is a first PD, and whereinthe free-space optics further comprise: a second PD; and a wavelengthdemultiplexer coupled between the PBS and the first and second PDs, thewavelength demultiplexer to separate a portion of the target returnsignal comprising light of a first wavelength to the first PD or thesecond PD.
 5. The free-space optics of claim 4, wherein the free-spaceoptics further comprise a polarization filter coupled between the PBSand the first and second PDs.
 6. The free-space optics of claim 1,wherein the lens system is a first lens system, wherein the optical beamcomprises a plurality of beams, and wherein the free-space opticsfurther comprise a second lens system coupled between an optical sourceof the LIDAR apparatus and the PBS to collimate the optical beam.
 7. Thefree-space optics of claim 1, wherein the LIDAR apparatus is afrequency-modulated continuous-wave (FMCW) LIDAR apparatus.
 8. A lightdetection and ranging (LIDAR) apparatus, comprising: an optical sourceto emit an optical beam; and free-space optics coupled with the opticalsource, comprising: a photodetector (PD); and a polarizing beam-splitter(PBS) to direct the optical beam in a first direction toward a targetenvironment; a polarization wave plate (PWP) to direct the optical beamtoward the target environment and to reflect a portion of the opticalbeam to the PBS for receipt by the PD, and wherein the reflected portionof the optical beam comprises a local oscillator signal (LO) to mix witha target return signal to generate target information; and a lens systemcoupled between the PBS and the PWP to magnify the optical beam.
 9. TheLIDAR apparatus of claim 8, wherein the optical beam emitted from theoptical source has a first polarization and the reflected portion of theoptical beam has a second polarization, the PWP to convert the opticalbeam from the first polarization to a third polarization and to convertthe target return signal from a fourth polarization to the secondpolarization.
 10. The LIDAR apparatus of claim 9, wherein the firstpolarization is a first linear polarization, the second polarization isa second linear polarization, the third polarization is a third circularpolarization, and the fourth polarization is a fourth circularpolarization.
 11. The LIDAR apparatus of claim 8, wherein the PD is afirst PD, and wherein the free-space optics further comprise: a secondPD; and a wavelength demultiplexer coupled between the PBS and the firstand second PDs, the wavelength demultiplexer to separate a portion ofthe target return signal comprising light of a first wavelength to thefirst PD or the second PD.
 12. The LIDAR apparatus of claim 11, whereinthe free-space optics further comprise a polarization filter coupledbetween the PBS and the first and second PDs.
 13. The LIDAR apparatus ofclaim 8, wherein the lens system is a first lens system, wherein theoptical beam comprises a plurality of beams, and wherein the free-spaceoptics further comprise a second lens system coupled between the opticalsource and the PBS to collimate the optical beam.
 14. The LIDARapparatus of claim 8, further comprising an optical scanner, coupledwith the PWP to illuminate the target environment, wherein a detectionaperture of the PD is larger than a diameter of the target return signalplus a de-scan error contributed by the optical scanner.
 15. A methodfor light detection and ranging (LIDAR) with free-space optics,comprising: generating an optical beam from an optical source having afirst polarization; directing the optical beam in a first direction,toward a polarizing beam splitter (PBS); propagating a first portion ofthe optical beam with the first polarization from the PBS for receipt bya photodetector (PD) as a local oscillator (LO) signal; directing asecond portion of the optical beam from the PBS through a lens system toa polarization wave plate (PWP) to convert the second portion of theoptical beam from the first polarization to a second polarization;receiving a target return signal from a target environment at the PWP toconvert the target return signal from a third polarization to a fourthpolarization; receiving, at the PBS through the lens system, the targetreturn signal having the fourth polarization; receiving, at the PD, aportion of the LO signal and a portion of the target return signal; andmixing the portion of the target return signal with the portion of theLO signal to generate target information.
 16. The method of claim 15,further comprising: removing optical noise at the PD with a polarizationfilter at the PD; maximizing a signal-to-noise ratio of the targetreturn signal with a PD aperture approximately equal to an opticalaperture of the free-space optics; and minimizing a de-scan error withthe PD aperture.
 17. The method of claim 15, wherein the firstpolarization is a first linear polarization and the second polarizationis a second circular polarization, and wherein the third polarization isa third circular polarization and the fourth polarization is a fourthlinear polarization.
 18. The method of claim 15, wherein the PWP is afirst PWP, and wherein the method further comprises: converting thetarget return signal from the fourth polarization to the secondpolarization with a second PWP; and reflecting the target return signal,with a reflector, through the PBS, wherein the target return signal isconverted from the second polarization to the first polarization. 19.The method of claim 15, wherein the PD is a first PD, and wherein themethod further comprises: separating a portion of the target returnsignal comprising light of a first wavelength utilizing a wavelengthdemultiplexer; and directing the portion of the target return signalcomprising the light of the first wavelength to the first PD or a secondPD.
 20. The method of claim 15, wherein the optical source is afrequency-modulated continuous-wave (FMCW) photonic integrated circuit(PIC).